Ubiquitin is a highly conserved 76-amino acid protein present in all eukaryotic cells (Callis and Vierstra (1989) Oxford Surv. Plant Mol. Cell. Biol. 6:1-30). Ubiquitin is encoded by small multigenic families containing two types of genes, polyubiquitin and ubiquitin extension/fusion genes. The polyubiquitin gene consists of tandem repeats of a 228 bp ubiquitin coding region (Burke, et al. (1988) Mol. Gen. Genet. 213:435-443; Binet, et al. (1991) Plant Mol. Biol. 17:395-407; Christensen, et al. (1992) Plant Mol. Biol. 18:675-689). The ubiquitin genes are translated as a polyprotein precursor, then proteolytically processed to functional ubiquitin monomers. The polyubiquitin genes are constitutively expressed in all plant organs that have been tested, with increased levels in young tissues (Binet, et al. (1991) Plant Sci. 79:87-94; Kawalleck, et al. (1993) Plant Mol. Biol. 21:673-684; Burke, et al. (1988) supra; Cornejo, et al. (1993) Plant Mol Biol Rep 14:19-21).
Promoters of polyubiquitin genes have been tested and reported to have high constitutive expression of foreign genes placed under their control. Such promoters have been isolated from Arabidopsis (Callis, et al., (1990) J. Biol. Chem. 265:12486-12493; Norris, et al. (1993) Plant Mol. Biol. 21:895-906), sunflower (Binet, et al. (1991) supra), tobacco (Genschik, et al. (1994) Gene 148:195-202), potato (Garbino and Belknap (1994) Plant Mol. Biol. 24:119-127), maize (Christensen, et al. (1992) supra), sugarcane (Wei, et al. (1999) J. Plant Physiol. 155:513-519) and rice (Wang, et al. (2000) Plant Sci. 156(2):201-211). Among these, the maize polyubiquitin Ubi1 promoter has been used in maize and other monocot transformation systems (Christensen, et al. (1992) supra; Cornejo, et al. (1993) supra; Christensen and Quail (1996) Transgenic Res. 5:213-218).